A mystery in the Catskills provides exciting summer days for an American family and a French family. Prof. Clovell's children, Steve and Hilda, are agog when the old chateau across the road is reopened by the Duvals. They make friends with Mavis Duval, but the plot thickens when a jewel thief starts prowling and makes a play for the diamond necklace, a long lost heirloom hidden in the Duval chateau. The kids help beat him to the draw, and for a still happier ending there's romance between the Clovell's newsman uncle and Mavis' Tante Riette. A sparkler.